See:How to clean cast iron– remove rust, reseason and refinish Over time, cast iron items will develop surface imperfections that add to their character, and make them look particularly in-keeping with a period home. When looked after, cast iron can last for centuries, but poorly maintained...
You can give it a hard refinish in the oven. Look up instructions for that. It'll give you perfect temperatures and the oils to use. And then just carefully season it every single time you use it and it'll be back to new. So those are your basics for everyday care of cast iron....
I commented how badly damaged it was and wished I knew how to remove the hideous white heat stains that marred the top. While talking I’m quickly covering the surface with a linen trying to hide the mess.